x86/proc: fix /proc/cpuinfo cpu offline bug

Impact: fix missing CPUs in /proc/cpuinfo after CPU hotunplug/hotreplug

In my test, I found that if a cpu has been offline,
the next cpus may not be shown in the /proc/cpuinfo.

if one read() cannot consume the whole /proc/cpuinfo,
c_start() will be called again in the next read() calls.
And *pos has been increased by 1 by the caller(seq_read()).
if this time the cpu#*pos is offline, c_start() will return
NULL, and the next cpus can not be shown.

this fix use next_cpu_nr(*pos - 1, cpu_online_map) to
search the next unshown cpu.

the most easy way to reproduce this bug:
1) offline cpu#1             (cpu#0 is online)
2) dd ibs=2 if=/proc/cpuinfo
   the result is that only cpu#0 is shown.
   cpu#2 and cpu#3 .... cannot be shown in /proc/cpuinfo
   it's bug.

Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

1 files changed, 4 insertions, 2 deletions

diff --git a/arch/x86/kernel/cpu/proc.c b/arch/x86/kernel/cpu/proc.c
index a26c480b9491..01b1244ef1c0 100644
--- a/arch/x86/kernel/cpu/proc.c
+++ b/arch/x86/kernel/cpu/proc.c
@@ -160,14 +160,16 @@ static void *c_start(struct seq_file *m, loff_t *pos)
 {
         if (*pos == 0)  /* just in case, cpu 0 is not the first */
                 *pos = first_cpu(cpu_online_map);
-        if ((*pos) < nr_cpu_ids && cpu_online(*pos))
+        else
+                *pos = next_cpu_nr(*pos - 1, cpu_online_map);
+        if ((*pos) < nr_cpu_ids)
                 return &cpu_data(*pos);
         return NULL;
 }
 
 static void *c_next(struct seq_file *m, void *v, loff_t *pos)
 {
-        *pos = next_cpu(*pos, cpu_online_map);
+        (*pos)++;
         return c_start(m, pos);
 }